A radio frequency (RF) resonant cavity thruster is a controversial proposed type of electromagnetic thruster where an anisotropic electromagnetic field inside the microwave cavity purportedly transfers momentum to the cavity producing thrust.

Conventional thrusters expel propellant, such as when ships move masses of water, aircraft move masses of air, or rockets expel exhaust products. A drive which does not expel propellant in order to produce a reaction force, but rather provides thrust from the electromagnetic field without any external interaction, is a reactionless drive. Such a closed system need not carry propellant and hence would be capable of always producing thrust, as long as it is powered, and would appear to violate the conservation of momentum and Newton's third law, leading many physicists to believe such thrusters to be impossible, labeling them as pseudoscience. Conversely, if the thrust is due to an interaction with an external field, the drive is then an open system, still propellant-less but not reaction-less, like a conventional spacecraft accelerates at the expense of the momentum of the planet it orbits during a gravity assist maneuver.

Despite the lack of theoretical consensus as to how such devices can work inventors continue to try to develop such drives because of the possibility of supporting long voyages in space, where propellant is a primary limiting factor. Roger Shawyer published a design with a tapered conical cavity, which he called the EmDrive. Guido Fetta later published a design with a pillbox cavity, which he called the Cannae Drive. A few groups of physicists have tried to build and test their own thrusters, based on the designs published by Shawyer and Fetta. Juan Yang at Xi'an's Northwestern Polytechnical University (NWPU) initially reported thrust, but retracted her claims in 2016 after a measurement error was identified and an improved setup measured no significant thrust. Harold White's group at NASA's Eagleworks Laboratories, which tests unusual rocket designs, tested a version of these designs. In 2015, they ran a test run that observed thrust of 40–100 μN from inputs of 40–80 W. Their paper was published in the Journal of Propulsion and Power.[9] At a press conference in Beijing on 10 December 2016 held by the China Academy of Space Technology (CAST) Dr Yue Chen, head of the communication satellite division at CAST, confirmed the agency is already testing an EmDrive in low Earth orbit on-board an "experimental verification platform". It stated it has successfully measured a thrust on-board laboratory conditions and plans to add the technology to Chinese satellites as soon as possible.